How to insert shell variable inside awk command - linux

Question

I'm trying to write a script, In this script i'm passing a shell variable into an awk command, But when i run it nothing happens, i tried to run that line only in the shell, i found that no variable expansion happened like i expected. Here's the code :
1 #!/bin/bash
2
3 # Created By Rafael Adel
4
5 # This script is to start dwm with customizations needed
6
7
8 while true;do
9 datestr=`date +"%r %d/%m/%Y"`
10 batterystr=`acpi | grep -oP "([a-zA-Z]*), ([0-9]*)%"`
11 batterystate=`echo $batterystr | grep -oP "[a-zA-Z]*"`
12 batterypercent=`echo $batterystr | grep -oP "[0-9]*"`
13
14 for nic in `ls /sys/class/net`
15 do
16 if [ -e "/sys/class/net/${nic}/operstate" ]
17 then
18 NicUp=`cat /sys/class/net/${nic}/operstate`
19 if [ "$NicUp" == "up" ]
20 then
21 netstr=`ifstat | awk -v interface=${nic} '$1 ~ /interface/ {printf("D: %2.1fKiB, U: %2.1fKiB",$6/1000, $8/1000)}'`
22 break
23 fi
24 fi
25 done
26
27
28 finalstr="$netstr | $batterystr | $datestr"
29
30 xsetroot -name "$finalstr"
31 sleep 1
32 done &
33
34 xbindkeys -f /etc/xbindkeysrc
35
36 numlockx on
37
38 exec dwm
This line :
netstr=`ifstat | awk -v interface=${nic} '$1 ~ /interface/ {printf("D: %2.1fKiB, U: %2.1fKiB",$6/1000, $8/1000)}'`
Is what causes netstr variable not to get assigned at all. That's because interface is not replaced with ${nic} i guess.
So could you tell me what's wrong here? Thanks.

Answer 1

If you want to /grep/ with your variable, you have 2 choices :
interface=eth0
awk "/$interface/{print}"
or
awk -v interface=eth0 '$0 ~ interface{print}'
See http://www.gnu.org/software/gawk/manual/gawk.html#Using-Shell-Variables

Answer 2

it's like I thought, awk substitutes variables properly, but between //, inside regex ( or awk regex, depending on some awk parameter AFAIR), awk variable cannot be used for substitution
I had no issue grepping with variable inside an awk program (for simple regexp cases):
sawk1='repo\s+module2'
sawk2='#project2\s+=\s+module2$'
awk "/${sawk1}/,/${sawk2}/"'{print}' aFile
(Here the /xxx/,/yyy/ displays everything between xxx and yyy)
(Note the double-quoted "/${sawk1}/,/${sawk2}/", followed by the single-quoted '{print}')
This works just fine, and comes from "awk: Using Shell Variables in Programs":
A common method is to use shell quoting to substitute the variable’s value into the program inside the script.
For example, consider the following program:
printf "Enter search pattern: "
read pattern
awk "/$pattern/ "'{ nmatches++ }
END { print nmatches, "found" }' /path/to/data
The awk program consists of two pieces of quoted text that are concatenated together to form the program.
The first part is double-quoted, which allows substitution of the pattern shell variable inside the quotes.
The second part is single-quoted.
It does add the caveat though:
Variable substitution via quoting works, but can potentially be messy.
It requires a good understanding of the shell’s quoting rules (see Quoting), and it’s often difficult to correctly match up the quotes when reading the program.